keith-turner commented on code in PR #4518:
URL: https://github.com/apache/accumulo/pull/4518#discussion_r1591458201


##########
server/tserver/src/main/java/org/apache/accumulo/tserver/BlockCacheMetrics.java:
##########
@@ -0,0 +1,57 @@
+/*
+ * Licensed to the Apache Software Foundation (ASF) under one
+ * or more contributor license agreements.  See the NOTICE file
+ * distributed with this work for additional information
+ * regarding copyright ownership.  The ASF licenses this file
+ * to you under the Apache License, Version 2.0 (the
+ * "License"); you may not use this file except in compliance
+ * with the License.  You may obtain a copy of the License at
+ *
+ *   https://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing,
+ * software distributed under the License is distributed on an
+ * "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
+ * KIND, either express or implied.  See the License for the
+ * specific language governing permissions and limitations
+ * under the License.
+ */
+package org.apache.accumulo.tserver;
+
+import org.apache.accumulo.core.metrics.MetricsProducer;
+import org.apache.accumulo.core.spi.cache.BlockCache;
+
+import io.micrometer.core.instrument.Gauge;
+import io.micrometer.core.instrument.MeterRegistry;
+
+public class BlockCacheMetrics implements MetricsProducer {
+
+  BlockCache indexCache;
+  BlockCache dataCache;
+  BlockCache summaryCache;
+
+  public BlockCacheMetrics(BlockCache indexCache, BlockCache dataCache, 
BlockCache summaryCache) {
+    this.indexCache = indexCache;
+    this.dataCache = dataCache;
+    this.summaryCache = summaryCache;
+  }
+
+  @Override
+  public void registerMetrics(MeterRegistry registry) {
+    Gauge.builder("indexCacheHitCount", indexCache, cache -> 
cache.getStats().hitCount())

Review Comment:
   If these are monotonically increasing counters, then it would be better to 
use a 
[FunctionCounter](https://docs.micrometer.io/micrometer/reference/concepts/counters.html#_function_tracking_counters)
 instead of a Gauge.  Not sure if they are, need to investigate.  The 
micrometer code to instrument caches uses FunctionCounter internally



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